This invention relates to a process for deactivating radicals formed prior to or during induced polymerization of dienes and/or vinyl aromatic compounds and, optionally, additional monomers.
During the radical-induced polymerization of butadiene in an emulsion, premature gel formation occurs when conversions are relatively high. In order to counteract this effect, which has a disadvantageous impact on the properties of the final elastomer thus obtained, it is necessary to stop the polymerization before the conclusion of the reaction. With other radical-reduced polymerizations, it is often desirable to interrupt the reaction prior to completion. Alkali metal or ammonium dialkyl dithiocarbamate, frequently used in combination with other polymerization inhibitors, has been employed in the past as stopping agents (see, e.g., Houben-Weyl, Methoden der Organischen Chemie, Vol. 14, p. 433 ff.). For example, the stopping effect of sodium dimethyl dithiocarbamate can be improved considerably if sodium nitrite is added simultaneously. However, the use of dithiocarbamate entails the substantial drawback that in the course of the acid processing of the latex nitrosamines are formed which, according to present-day knowledge, must be considered a health hazard [see, e.g., Umschau 1985 (1), 24]. This risk is further increased if simultaneously sodium nitrite is employed. Accordingly, efforts have been made to select reaction conditions which, from the outset, preclude the formation of nitrosamines [see, e.g., Gummi, Asbest, Kunststoffe 39, 6 (1986)]. A similar problem arises if the undesired polymerization of radical-forming monomers is to be inhibited. For this purpose, as well, carcinogenic substances are frequently employed according to the state of the art, sometimes in appreciable quantities.
Amine-free polymerization inhibitors are known in principle. A summary is to be found, e.g., in the above-cited standard textbook by Houben-Weyl. However, those inhibitors suffer from the general disadvantage that the vulcanization rate of the rubbers thus obtained is clearly lower than that of comparable caoutchoucs stopped with the above mentioned system dimethyldithiocarhamate with or without sodium nitrite (see, e.g., U.S. Pat. No. 3,222,334).